Andrew, I'm building a 64 300(aluminum heads) for my son; for the street on pump gas. I assume, since yours is a 66 300, it has iron heads. I don't know how much money you want to spend, so I'll just cover the basics. You really can't spend too much money because the 300's design has some limits and there are some parts availability issues.
1]Bore the block .050" over and install Buick 350 pistons. Several CR's are available.
2]Get a stock Buick 300 aluminum 4bbl intake manifold, it's the only one available.
3]Since the heads have shaft rockers, a radical cam is not going to work well with them. Choose a cam with a lift of no more than .500" and the biggest area under the torque/hp curve. An rpm range of off idle to 5500 rpm will work. If you plan to go over 5500rpm, your going to have to increase the valve diameters and do some flow work on the heads and intake manifold. SBC hi-pro valves springs will work too.
4] The stock manifold will accept a 600-625 cfm(?) Carter AFB.
5]The exhaust manifolds are a pretty good design, but get headers if you can find them. Use at least a 2" dia. dual exhuast system with an "H" or "X" pipe. A 2 1/4" or 2 1/2" dia. system would ideal.
6]Install a breakerless ignition with hi-volt coil.
7]If you have an auto trans, get a 3.08 or 3.23:1 rear gear. If 4 speed go higher but not over 3:73:1.
